1. Field of the Invention
The invention relates to a method for the formation of 3-alkylthiophenes or 3-arylthiophenes from 3-halothiophenes. More particularly, the invention pertains to improvements on the Kumada coupling reaction for the production of 3-alkylthiophenes or 3-arylthiophenes.
2. Description of the Related Art
Alkyl and aryl substituted thiophenes are important intermediates for conductive polymers. Conducting polymers are materials that possess the electrical properties of metals yet retain the mechanical properties of polymers. Such conductive polymers are materials that are made conductive when combined with a doping material that facilitates polymer conductivity. This is commonly referred to in the art as “doping the polymer” and provides a lower energy threshold for conductivity. Doping materials suitable for doping of some conductive polymers include halogens such as iodine, bromine and chlorine. Recent years have lead to the development of conductive polymers that are even able to approach the conductivity of naturally conductive metals. Today, conductive polymers are particularly desirable materials for the fabrication of devices such as optical and electronic devices, electroluminescent devices, sensors and shielding materials.
The degree of conductivity exhibited by conductive polymers depends on the degree of order on a molecular level. This is due in part to the crystal lattice that allows an overlapping pathway for electrons. Included among polymers that have shown conductive properties when combined with appropriate doping materials are polythiophenes. Polythiophenes are particularly desirable because they effectively self-assemble into well-ordered, highly conducting nanoscale layers and have versatile properties that renders them useful for a wide range of commercial applications. However, one disadvantage of poly(thiophenes) is that they are not soluble, making them difficult to process. In order to increase the solubility and processability it is known to add alkyl chains in the 3-position, thereby obtaining a poly(3-alkylthiophene).
For example, in the article “Nickel-Phosphine Complex-Catalyzed Grignard Coupling II. Grignard Coupling of Heterocyclic Compounds” by Tamao, K.; Kodama, S.; Nakajima, I.; Kumada, M.; Minato, A.; and Suzuki, K (Tetrahedron 1982, 38, 3347–3354) (herein after “Kumada, et al.”), cross-coupling reactions of heterocyclic halides with various Grignard reagents in the presence of nickel-phosphine complexes as catalysts are discussed. Particularly, the article discusses methods for introducing organic groups, e.g. alkyl groups, into halogenated heterocycles, such as five- and six-membered nitrogen or sulfur-containing heterocyclic compounds, using a nickel-phosphine catalyst complexes, such as [1,3-bis(diphenylphosphonyl)propane nickel(II) chloride] (NiCl2dppp). According to Kumada, et al., reaction procedures are described teaching the introduction of an organic group onto the carbon atom of the heterocycle to which the halogen has been attached, giving an isomerically pure coupling product. The procedures described by Kumada, et al., however, have been found to generate an undesirable dithienyl byproduct that precipitates from reaction mixtures, thus limiting the desired product yield.
The present invention is an improved process for the coupling reactions described by Kumada, et al. The present invention provides a process for the preparation of 3-alkylthiophenes and 3-arylthiophenes at an exceptionally high yield.
Specifically, the invention describes a method for forming a 3-alkylthiophene or 3-arylthiophene which comprises reacting a 3-halothiophene with an alkylmagnesiumhalide Grignard reagent in the presence of a catalyst and a 2-methyl tetrahydrofuran solvent having a reagent concentration of at least about 0.5 mol/L in said solvent. It has unexpectedly been found that the use of a 2-methyl tetrahydrofuran solvent allows for higher concentrations of the Grignard reagent with minimal or no dithienyl side product generation. The resulting high yields (space yield, kg/l) of the desired 3-alkylthiophene reaction product are about five times the yield compared to the well known process described by Kumada, et al. The crude yield determined by gas chromatography of the reaction mixture or selectivity could be increased from about 70–80% up to about 97–99%.